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Dephlegmators are widely used in the process industries for the separation of gas mixtures, particularly those which contain components with sub-ambient boiling points. Such separations require significant amounts of low temperature refrigeration and are thus highly energy intensive. Dephlegmators offer simple, reliable, and efficient operation for such gas separations.
The characteristic feature of dephlegmator operation is the utilization of simultaneous heat and mass transfer in a group of generally vertical flow channels or passageways in indirect heat transfer communication with other flow channels containing heating or cooling fluids. A dephlegmator thus combines both heat transfer and mass transfer in a single operating system. Heat and mass transfer in process streams within dephlegmator channels can occur in either a condensation or vaporization mode.
In the condensation or rectification mode of operation, a feed gas mixture is cooled and partially condensed within a group of flow channels by indirect heat transfer with one or more refrigerants or colder fluids flowing in adjacent channels. The resulting condensed liquid flows downward while exchanging heat and mass with the remaining vapor, which flows upward. A liquid stream enriched in higher boiling components and a vapor stream enriched in lower boiling components are withdrawn from the feed flow channels. Rectification occurs in this operation, and a dephlegmator operating in this mode is often called a rectifying condenser or rectifying dephlegmator. This type of dephlegmator can be used for rejecting nitrogen from natural gas (U.S. Pat. Nos. 4,732,598 and 5,802,871), producing refrigerated liquid methane (U.S. Pat. No. 5,983,665), recovering helium from natural gas (U.S. Pat. Nos. 5,017,204 and 5,329,775), purifying synthesis gas (U.S. Pat. No. 4,525,187), recovering C4+ hydrocarbons (U.S. Pat. No. 4,519,825), and for recovering olefins from hydrocarbon-hydrogen mixtures such as cracked gases, refinery offgases, and petrochemical plant offgases (U.S. Pat. Nos. 5,361,589, 5,377,490, 5,379,597, and 5,634,354).
In the vaporization or stripping mode of operation, a liquid feed mixture is heated and partially vaporized within a group of flow channels by indirect heat transfer with one or more warmer fluids flowing in adjacent channels. The vaporizing liquid flows downward while exchanging heat and mass with the generated vapor, which flows upward. Stripping action is promoted by the upward flowing vapor. A liquid stream enriched in higher boiling components and a vapor stream enriched in lower boiling components are withdrawn from the feed channels. This type of dephlegmator, often called a stripping dephlegmator, is described in representative U.S. Pat. No. 5,596,883.
Some condensing type dephlegmators utilize an upward-flowing boiling liquid to provide refrigeration in a group of flow channels which remove heat by indirect heat exchange from a condensing stream in adjacent channels. The refrigeration channels are open at the lower end, and usually at the upper end as well, and the dephlegmator may be partly or completely submerged in the boiling liquid. This type of refrigeration circuit is called a thermosiphon heat exchanger and is discussed further below.
A combined mode of operation also is possible in which a vapor is condensed in a first group of flow channels while a liquid is vaporized in a second group of channels, wherein the first and second groups of channels are in heat transfer communication. Heat to vaporize the liquid in the second group of channels is provided by the condensing vapor in the first group of channels, rectification occurs in the first group of channels, and stripping occurs in the second group of channels. This type of dual-mode dephlegmator is used for air separation as described in U.S. Pat. Nos. 5,592,832 and 5,899,093.
Dephlegmators are constructed with multiple flow channels or passageways which are grouped and manifolded to segregate process stream(s) from heating or cooling stream(s) while allowing indirect heat transfer between the streams. More than two groups of channels can be used to process multiple streams in the same dephlegmator. Plate and fin heat exchangers, also known as core-type exchangers, are widely preferred for dephlegmator service. These are typically of brazed aluminum construction, but any appropriate metals can be used. Shell and tube heat exchangers have utility as dephlegmators, but are less favored than the plate and fin configuration.
In the operation of dephlegmators used for the separations described above, the proper distribution of the process feed stream into the multiple flow channels and the withdrawal of vapor and/or liquid product streams from the multiple flow channels are necessary for efficient operation. Of particular importance in a widely-used type of condensing dephlegmator described below is the proper introduction of feed gas into the bottom end of a group of flow channels while withdrawing condensate from the bottom end of the same flow channels.
Several methods have been proposed to introduce feed vapor into and remove condensed liquid from the bottom of a brazed aluminum, core-type dephlegmator. U.S. Pat. Nos. 5,333,683, 3,992,168, 3,983,191 and 3,612,494 disclose the use of two separate headers, one for the vapor to enter the bottom of the dephlegmator core and the other for the liquid to drain from the bottom of the core. These designs require distribution fins, both to distribute the vapor into the core and to collect the liquid draining from the core. These distribution fins, particularly the vapor distribution fins, reduce the fluid-handling capacity of the core below that which could otherwise be attained in the full cross-section of heat/mass transfer flow channels used in the main body of the dephlegmator core.
U.S. Pat. Nos. 5,144,809, 3,568,462 and 3,568,461 show the use of integral dome headers which enclose the entire bottom end of the dephlegmator core and allow vapor to enter the core and liquid to drain from the core without obstruction. However, to have adequate mechanical strength, these dome headers are restricted to relatively low pressure applications or cores of relatively small cross-section.
Other methods have been proposed to separate vapor and liquid exiting a conventional core-type heat exchanger or for the input or output of fluids from core-type heat exchangers.
U.S. Pat. Nos. 5,765,631, 5,321,954 and 4,599,097 show various types of integral domes and other integrated vessels which can be used primarily to separate mixtures of vapor and liquid entering or leaving a conventional core-type heat exchanger in order to individually distribute them into the core or remove them from the core. Some of these devices alternatively could be used for input or output of fluids from dephlegmator cores, but they are also restricted to use in relatively low-pressure applications or with cores of relatively small cross-section.
U.S. Pat. No. 5,385,203 discloses a conventional core-type heat exchanger mounted inside a partitioned vessel such that the several separate chambers formed by the partitions provide a multi-stage thermosiphon-type heat exchanger with different boiling refrigerants in each of the separate chambers. Circulation of the boiling refrigerants is obtained by the submergence of appropriate sections of the core in the refrigerant liquids contained within each of the chambers. The thermosiphon boiling refrigerants in the open circuits of the core serve to cool a process gas stream contained within a closed circuit of the core.
Integral domes and other vessels mounted on a conventional core-type heat exchanger as shown in U.S. Pat. No. 4,330,308 provide a similar multi-stage thermosiphon-type heat exchanger with different boiling refrigerants in each of the separate sections of the core. Circulation of the boiling refrigerants is obtained by the submergence of appropriate sections of the core in the refrigerant liquids contained within each of the sections of the core. Other dome-type integrated vessels are shown to introduce a vapor/liquid refrigerant mixture into the core heat exchanger. These devices are also restricted to use in relatively low-pressure applications or with cores of relatively small cross-section.
These thermosiphon-type heat exchanger core assemblies are analogous to a series of kettle-type shell and tube heat exchangers used to cool a process stream in the tube circuit by means of a boiling refrigerant in the enlarged, or kettle-type, shell. Altec International, La Crosse, Wis., manufactures similar brazed aluminum Core-in-Kettle(trademark) heat exchangers for use in place of kettle-type shell and tube heat exchangers.
U.S. Pat. Nos. 5,071,458 and 4,606,745 describe air separation plant reboiler-condenser core-type heat exchangers which are installed inside distillation columns. These cores are at least partially submerged in liquid oxygen refrigerant to provide the driving force for the thermosiphon boiling of the oxygen in a low pressure column, typically operating below 30 psia, which serves to condense nitrogen vapor from a higher pressure column.
The efficient operation of core-type dephlegmators requires that the feed gas mixture entering a group of flow channels be evenly distributed so that the entire cross-section of the dephlegmator is fully utilized. Maldistribution will reduce the efficiency of a dephlegmator, thereby decreasing the degree of separation.
In a rectifying core-type dephlegmator which operates in the condensing mode, feed gas is introduced into the bottom end of a group of flow channels while condensate is withdrawn from the bottom end of the same flow channels. In the prior art described above, headers and distributor devices are required for the distribution of feed gas and collection of condensed liquid. The present invention described and defined below is an improved dephlegmator design which does not require headers and distributors at the lower end, and optionally at the upper end, of the dephlegmator core. This promotes efficient utilization of the entire core cross-section for heat and mass transfer without the flow restrictions caused by distributors and headers.
The invention is a system for the separation of a feed gas mixture containing at least one more volatile component and at least one less volatile component, which system comprises:
(a) a pressure vessel having an interior and an exterior;
(b) a dephlegmator installed in the interior of the pressure vessel, wherein the dephlegmator comprises a group of flow passageways, each passageway having an upper end and a lower end, and wherein the lower ends of the flow passageways are open and are in flow communication with the interior of the pressure vessel;
(c) at least one vapor header in flow communication with the upper ends of the flow passageways, and piping means for withdrawing a vapor product enriched in the more volatile component from the vapor header to the exterior of the pressure vessel;
(d) piping means for introducing the feed gas mixture into the interior of the pressure vessel; and
(e) piping means for withdrawing from the interior of the pressure vessel a liquid product enriched in the less volatile component.
The system can further comprise:
(f) one or more additional groups of flow passageways in the dephlegmator wherein each of the flow passageways has an upper end and a lower end, and wherein the group of additional flow passageways is in indirect heat transfer communication with the group of flow passageways of (b);
(g) an upper header in flow communication with the upper ends of the flow passageways of (f) and a lower header in flow communication with the lower ends of the flow passageways of (f); and
(h) piping means for introducing refrigerant from the exterior of the pressure vessel into one header of (g) and piping means for withdrawing refrigerant from the other header of (g) to the exterior of the pressure vessel.
The dephlegmator can be constructed in a plate and fin configuration or in a shell and tube configuration.
Optionally, the system can further comprise one or more additional dephlegmators installed in the pressure vessel and configured to operate in parallel with the dephlegmator of (b) above.
In another embodiment, the system can further comprise:
(f) an additional pressure vessel having an interior and an exterior;
(g) an additional dephlegmator installed in the interior of the additional pressure vessel, wherein the additional dephlegmator comprises a group of flow passageways, each passageway having an upper end and a lower end, and wherein the lower ends of the flow passageways are open and are in flow communication with the interior of the additional pressure vessel;
(h) at least one vapor header in flow communication with the upper ends of the flow passageways, and piping means for withdrawing a vapor product further enriched in the more volatile component from the vapor header to the exterior of the additional pressure vessel;
(i) piping means for transferring the vapor product of (c) from the pressure vessel of (a) into the interior of the additional pressure vessel of (f); and
(j) piping means for withdrawing from the interior of the additional pressure vessel an additional liquid product enriched in the less volatile component.
In an alternative embodiment, the system can further comprise:
(k) one or more groups of additional flow passageways in the additional dephlegmator wherein each of the flow passageways has an upper end and a lower end, and wherein the group of additional flow passageways is in indirect heat transfer communication with the group of flow passageways of (g);
(l) an upper header in flow communication with the upper ends of the flow passageways of (k) and a lower header in flow communication with the lower ends of the flow passageways of (k); and
(m) piping means for introducing refrigerant from the exterior of the additional pressure vessel into one header of (1) and piping means for withdrawing refrigerant from the other header of (1) to the exterior of the additional pressure vessel.
The invention also includes a system for the separation of a feed gas mixture containing at least one more volatile component and at least one less volatile component, which system comprises:
(a) a pressure vessel having an interior and an exterior;
(b) a dephlegmator installed in the interior of the pressure vessel, wherein the dephlegmator comprises a group of flow passageways, each passageway having an upper end and a lower end, and wherein the upper and lower ends of the flow passageways are open and are in flow communication with the interior of the pressure vessel;
(c) seal means disposed in the pressure vessel at an axial location between the upper and lower ends of the flow passageways wherein the seal means divides the interior of the pressure vessel into an upper section and a lower section which are not in flow communication, wherein the upper ends of the flow passageways are in flow communication with the upper section of the pressure vessel and the lower ends of the flow passageways are in flow communication with the lower section of the pressure vessel;
(d) piping means for introducing the feed gas mixture into the lower section of the pressure vessel;
(e) piping means for withdrawing a vapor product enriched in the more volatile component from upper section of the pressure vessel; and
(f) piping means for withdrawing from the lower section of the pressure vessel a liquid product enriched in the less volatile component.
The system can further comprise;
(g) one or more additional groups of flow passageways in the dephlegmator wherein the each of the flow passageways has an upper end and a lower end, and wherein the group of additional flow passageways is in indirect heat transfer communication with the group of flow passageways of (b);
(h) an upper header in flow communication with the upper ends of the flow passageways of (g) and a lower header in flow communication with the lower ends of the flow passageways of (g); and
(i) piping means for introducing refrigerant from the exterior of the pressure vessel into one header of (h) and piping means for withdrawing refrigerant from the other header of (h) to the exterior of the pressure vessel.
The dephlegmator can be constructed in a plate and fin configuration or in a shell and tube configuration.
Alternatively, the system can further comprise an additional dephlegmator installed in the pressure vessel and configured to operate in parallel with the dephlegmator of (b).
In another embodiment, the system can further comprise:
(g) an additional pressure vessel having an interior and an exterior;
(h) an additional dephlegmator installed in the interior of the additional pressure vessel, wherein the dephlegmator comprises a group of flow passageways, each passageway having an upper end and a lower end, and wherein the upper and lower ends of the flow passageways are open and are in flow communication with the interior of the pressure vessel;
(i) seal means disposed in the additional pressure vessel at an axial location between the upper and lower ends of the flow passageways, which seal means divides the interior of the additional pressure vessel into an upper section and a lower section which are not in flow communication, wherein the upper ends of the flow passageway are in flow communication with the upper section of the pressure vessel and the lower ends of the flow passageways are in flow communication with the lower section of the pressure vessel;
(j) means for transferring the vapor product of (e) from the upper section of the pressure vessel into the lower section of the additional pressure vessel;
(k) piping means for withdrawing a vapor product further enriched in the more volatile component from upper section of the additional pressure vessel; and
(l) piping means for withdrawing from the lower section of the additional pressure vessel a liquid product enriched in the less volatile component.
The invention also is a method for the separation of a feed gas mixture containing at least one more volatile component and at least one less volatile component which comprises:
(a) providing a pressure vessel having an interior and an exterior;
(b) providing a dephlegmator installed in the interior of the pressure vessel, wherein the dephlegmator comprises a group of flow passageways, each passageway having an upper end and a lower end, and wherein the lower ends of the flow passageways are open and are in flow communication with the interior of the pressure vessel;
(c) introducing the feed gas mixture into the interior of the pressure vessel;
(d) passing the feed gas mixture upwardly through the flow passageways and condensing therein at least a portion of the less volatile components by indirect heat transfer with one or more refrigerants, wherein the condensate so formed flows downward in heat and mass transfer relation with upward flowing vapor and collects in the bottom of the pressure vessel;
(e) providing at least one vapor header in flow communication with the upper ends of the flow passageways and withdrawing a vapor product enriched in the more volatile component from the vapor header to the exterior of the pressure vessel; and
(f) withdrawing from the interior of the pressure vessel a liquid product enriched in the less volatile component.
The feed gas can comprise two or more components selected from the group consisting of hydrogen, helium, nitrogen, carbon monoxide, carbon dioxide, oxygen, and hydrocarbons having from one to six carbon atoms.
In another embodiment, the invention is a method for the separation of a feed gas mixture containing at least one more volatile component and at least one less volatile component which comprises:
(a) providing a pressure vessel having an interior and an exterior;
(b) providing a dephlegmator installed in the interior of the pressure vessel, wherein the dephlegmator comprises a group of flow passageways, each passageway having an upper end and a lower end, and wherein the upper and lower ends of the flow passageways are open and are in flow communication with the interior of the pressure vessel;
(c) providing seal means disposed in the pressure vessel at an axial location between the upper and lower ends of the flow passageways wherein the seal means divides the interior of the pressure vessel into an upper section and a lower section which are not in flow communication, wherein the upper ends of the flow passageways are in flow communication with the upper section of the pressure vessel and the lower ends of the flow passageways are in flow communication with the lower section of the pressure vessel;
(d) introducing the feed gas mixture into the lower section of the pressure vessel;
(e) passing the feed gas mixture upwardly through the flow passageways and condensing therein at least a portion of the less volatile component by indirect heat transfer with one or more refrigerants, wherein the condensate so formed flows downward in heat and mass transfer relation with upward flowing vapor and collects in the bottom of the pressure vessel;
(f) withdrawing a vapor product enriched in the more volatile component from upper section of the pressure vessel; and
(g) withdrawing a liquid product enriched in the less volatile component from the lower section of the pressure vessel.
The feed gas ion this embodiment can comprise two or more components selected from the group consisting of hydrogen, helium, nitrogen, carbon monoxide, carbon dioxide, oxygen, and hydrocarbons having from one to six carbon atoms.